PROJECT SUMMARY/ABSTRACT Radiation therapy plays a central role in the treatment of cancer. The delivery of safe and effective radiation hinges on the ability to identify the radiation target (tumor), and areas to avoid (normal tissues). The act of defining the radiation target ? referred to as contouring ? requires a sound understanding of anatomy, radiology, and most importantly the individual patient. Existing research clearly shows that poor contouring leads to increased toxicity, and decreased survival, which emphasizes the importance of contouring to deliver safe and effective radiation therapy. This project proposes to create and test a simulation-based educational approach to improving contour quality in radiation oncology. This contouring simulation tool will allow radiation oncology providers the ability to contour test cases and receive rapid feedback on the accuracy of their contours. We will use knowledge-based planning to help provide feedback on the real life risks of toxicity and chances of cure had their contours been used to treat that patient. Using iterative and participatory design we will engage stakeholders to develop a comprehensive interactive web-based simulation tool (iContour), as well as a more focused mobile application (iContour-mobile). Successful educational applications require user engagement, therefore to increase user engagement we will leverage components of gamification and social media. With a randomized clinical trial we will test the efficacy of these different simulation strategies on real patient contours. Overall, this study seeks to define the optimal simulation strategy of how to improve real-world contouring. If successful this simulation-based education approach could transform contouring education and play a central role in residency education, continuing medical education, and board certification for radiation oncology, and serve as a model for developing simulation training tools for other fields in medicine.